I am in the process of adding a sump with an ATS, meanwhile I have a CPR HOB sump packed like a brick with cheato. Since they will get flow equally from the DT, I am presuming both will work , albeit competing against each other - along with the corals and algae.
If they are sequential- the first one in line of the flow is sure to out compete the second.
Also, ATS can be sized to how much you feed your fish (not water volume ) and you can put it on a timer.
I want both since the cheato is a safe haven for the pods which eventually feed the mandarin.
I'll be posting the progress in the nano forum.